1. Field of the Invention
The present invention relates to a method and an electronic control circuit, such as for example an electronic control gear, for regulating the operating behaviour, in particular the brightness, of gas discharge lamps.
2. Description of the Related Art
FIG. 5 shows, for example, the construction of a known electronic control gear for controlling a gas discharge lamp 3 as disclosed in EP-A1-0 490 329. EP-A1-0 338 109 has disclosed an electronic control gear similar to the circuit arrangement shown in FIG. 5 in which however, in place of heating transformers T1 and T2 illustrated in FIG. 5, an ignition or heating capacitor connected in parallel to the lamp coils is used.
In accordance with FIG. 5, a rectifier 1, comprising diodes D1-D4 connected to form a bridge circuit, is connected via capacitors C1 and C2 to an a.c. voltage source u.sub.E. The capacitors C1 and C2 are part of a radio interference suppression arrangement. The input a.c. voltage u.sub.E rectified by the rectifier 1 is fed to an inverter 2 which generally comprises two alternately switched semiconductor switches. The inverter 2 converts the line voltage rectified by the rectifier 1 into an output a.c. voltage. Here the output frequency and/or duty factor between the switch-on times of the semiconductor switches of the inverter 2 is/are variable. The output voltage of the inverter is fed to a load circuit comprising a series-resonance circuit consisting of a coil Li and a capacitor C8, a coupling capacitor C4, heating transformers T1 and T2 for the lamp coils, and the gas discharge lamp 3. The gas discharge lamp 3 is connected via wire conductors to heating transformers T1 and T2. The lamp current i.sub.L flowing across the gas discharge path of the gas discharge lamp 3 is tapped across a shunt resistor R1 and is normally used as regulating variable for the brightness of the gas discharge lamp 3, i.e. the frequency and/or duty factor of the inverter 2 is regulated as a function of the actual value of the lamp current i.sub.L in order to dim the brightness of the gas discharge lamp 3. The gas discharge lamp 3 is dimmed in that the output frequency f of the inverter 2 is increased. The initial ignition of the gas discharge lamp 3 takes place in that the output frequency of the inverter 2 is adapted to the resonance frequency of the series-resonance circuit comprising the coil L1 and the capacitor C8.
If, however, the lamp current i.sub.L is used as regulating variable, operating disturbances can occur due to excessively long connection lines between the terminals of the electronic control gear and the gas discharge lamp. This manifests in particular in the case of strong dimming, i.e. with low brightness of the gas discharge lamp. These operating disturbances are caused by capacitive influences of the wiring as parasitic capacitances C5 and C6 occur between the lines and earth and a parasitic capacitance C7 occurs between the lines. It can be seen from FIG. 5 that the capacitances C5 and C6 between the lines and earth only slightly in fluence the lamp current i.sub.L measured via the resistor R1 as the capacitive currents flowing across these capacitances C5 and C6 are conducted past the resistor R1 and the capacitance C6 of the line 2 is balanced to earth by the dimming c apacitor C3 relative to R1.
On the other hand, the capacitive current produced by the parasitic capacitance C7 between the lines is superimposed upon the lamp current i.sub.L measured via the shunt resistor R1. The resistance equivalent circuit diagram, shown in FIG. 6, of the lines with the gas discharge lamp 3 and the parasitic capacitance C7 shows that the shunt resistor R1 is supplied via the line 2 with the lamp current i.sub.L flowing across the lamp resistor R.sub.Lampe and the capacitive current i.sub.C7 flowing across the parasitic capacitance C7. Thus not a purely ohmic current, but a current i.sub.L +i.sub.C7, shifted in phase relative to the lamp voltage as a function of the parasitic capacitance C7, flows across the shunt resistor R1.
As already mentioned, the current flowing across the resistor R1 is used to regulate the brightness of the gas discharge lamp 3. Here in particular the peak value of this current is measured, which peak value is compared with a predetermined setpoint value which is variable by means of the dimming setting, whereupon optionally the output frequency f or the duty factor d of the switches of the inverter of the electronic control gear is changed. If too high a lamp current is detected via the resistor R1, the inverter frequency f is increased so that the voltage across the heating capacitor C8 of the series oscillating circuit falls. In this case the gas discharge lamp connected in parallel with the capacitor C8 is connected to a lower voltage and thus emits less light.
However, in the case of the known circuit arrangement shown in FIG. 5 accurate regulation of the operating behaviour of the gas discharge lamp as a function of the lamp current measured via the resistor R1 is not possible, as the peak value of the sum current i.sub.L (t)+i.sub.c.7 (t), (compare FIG. 6) i.e. the sum of the lamp current iL flowing across the coil resistors R.sub.Wendel 1 and R.sub.Wendel 2 and the lamp resistor R.sub.Lampe of the gas discharge lamp 3 and the capacitive current i.sub.C7 flowing across the parasitic capacitance C7, is actually measured via the resistor R1. As with increasing frequency, i.e. stronger dimming, the capacitive resistance of the parasitic capacitance C7 falls and the lamp resistance R.sub.Lampe remains constant, in the current measured via the resistor R1 the capacitive current component i.sub.C7 increases in relation to the purely ohmic lamp current component i.sub.L. This means that, with strong dimming, the electronic control gear detects an excessively high current across the resistor R1 and incorrectly interprets this excessive current as actual value of the lamp current i.sub.L.
This process is illustrated in FIG. 8, in which FIGS. 8a to 8c show different characteristics of the lamp voltage u.sub.L and the current i.sub.R1 measured across the resistor R1, for different values of the line capacitance C7. FIG. 8a illustrates the ideal situation in which the parasitic line capacitance C7 is very small, so that the capacitive current component i.sub.C7 of the current measured across the resistor R1 is negligibly small and this current measured across the resistor R1 substantially corresponds to the lamp current i.sub.L. As in this case the current is substantially purely ohmic, the current is not shifted in phase compared to the lamp voltage u.sub.L. As already described, usually the peak value of the current detected across the resistor R1 is measured. This peak value is compared, as actual value list, with a predetermined setpoint value I.sub.soll In the example illustrated in FIG. 8a the measured peak value I.sub.ist corresponds to the setpoint value I.sub.soll and therefore no regulation of the brightness of the gas discharge lamp 3 is required. FIG. 8b illustrates this process when an average line capacitance C7 occurs between the lines. It can be seen from FIG. 8b that, as a result of the capacitive current i.sub.C7 flowing across the line capacitance C7, not only is the current i.sub.R1 also shifted in phase relative to the lamp voltage u.sub.L but the measured peak value list is also distinctly increased compared to the situation illustrated in FIG. 8a. Therefore in the case of the signal characteristic illustrated in FIG. 8b the electronic control gear would recognise that the measured peak value I.sub.ist, is greater than the measured setpoint value I.sub.soll The electronic control gear would therefore attempt to reduce this increased current again by increasing the frequency across the inverter 2, although this is opposed by the capacitive resistance--falling due to the rising frequency--of the parasitic capacitance C7, with the result that the capacitive component i.sub.C7 of the current i.sub.R1 measured across the shunt resistor R1 is increased. This cycle finally leads to the extinguishing of the lamp, although the extinguishing of the gas discharge lamp 3 by the electronic control gear cannot be ascertained by measurement of the current i.sub.R1 flowing across the resistor R1 as a capacitive current i.sub.C7 also flows when the lamp is extinguished via the parasitic capacitance C7. FIG. 8c illustrates corresponding signal characteristics for the occurrence of a high line capacitance value between the lines which connect the gas discharge lamp 3 to the electronic control gear. In the case of the signal characteristics illustrated in FIG. 8c, due to the distinctly increased capacitive current component i.sub.C7 relative to the lamp current i.sub.L, the resultant sum current i.sub.R1 is distinctly increased again. The measurement error occurring as a result of the superimposition with the capacitive current i.sub.C7 is greatest in the example illustrated in FIG. 8c.
It will be apparent from the previous description that due to the capacitive influences of the wiring, correct dimmer operation of the gas discharge lamp is not possible since, particularly in the case of strong dimming of the gas discharge lamp, the capacitive current i.sub.C7 flowing across the line capacitance C7 is distinctly superimposed upon the lamp current i.sub.L actually to be measured. The previously employed method, in which the peak value of the current i.sub.R1 flowing across the resistor R1 is measured, is therefore too inaccurate in the case of electronic control gears covering wide dimming ranges (between 100% and 1% brightness). For different cable types and different cable lengths l and for different operating frequencies f, FIG. 7 shows the value of the parasitic line capacitance C7 occurring between the lines. It can be seen from FIG. 7 that on the one hand the parasitic capacitance C7 rises with increasing cable length l and on the other hand a larger line capacitance occurs at a lower operating frequency f. The measurement results shown in FIG. 7 indicate that, for the line capacitance to have a small influence, light manufacturers would need to take into account a specified, maximum permissible length of the wiring. However, presetting of a maximum permissible wiring length is undesirable.
Therefore the object of the invention is to provide a method and a control circuit, in particular for the implementation of the method, for regulating and/or measuring the operating state of gas discharge lamps in which accurate regulation and/or measurement of the operating state is possible and it is unnecessary to take into account the wiring length between the gas discharge lamp and a series-connected electronic control gear.
In accordance with the invention it is proposed that only the active component of the lamp current be evaluated. In this way the influence of the capacitive current flowing across the parasitic line capacitance is eliminated and accurate regulation and/or measurement of the operating state is possible without the need to take the line length into account. In particular, the connected lamp can be accurately dimmed over wide dimming ranges.
Further advantageous developments of the invention are described hereinafter.